Firefox 115.27 ESR works fine here. Then again I also use Supermium browser on it too. the only time I ever had a problem with windows 7 from 2007 until present day, was when I blocked myself out of my own system by accidently turning off the workstation windows service (talk about drama)
There is no such thing as safty online at ALL, only the remediation of it. If a hacker wants in. they will find a way. If data goes into your computer then it can also go out of your computer. firewalls work to a point, but never fully and in most cases the default windows 7 firewall works better then any "modern one" -minus one or two like Windows firewall control 5 (not the malware bytes version). GPEdit.msc is your friend on older systems. along with removing or masking certain windows built in .exe programs out of the Windows32 directory, but if thats if your not installing anything else anymore and your system is the way you like it..
"Tweaking windows 7" and hardening it is the best option even with an old browser.
Theres no such thing as "security" its always been an illusion. if man can make it, he can also break it. and programmers rule of thumb "always keep a backdoor" so, know your system, know your OS and you can still use windows 7 another 30 years (provided some open source browsers keep going with their projects)
GPEdit.msc in this case is your friend (but you can lock yourself out of your own system so use with caustion)
If you dont use any remote desktop services, shut them off or any remote software. also remove or hide all Ras dial and remote exe.x in windows32 directory.
(all you can do is limit the chances of getting hacked or accessed to a point. Its never fool proof)
And as for security well. between your ISP, your router, your settings and all the misc servers and server hops to what ever site your going to. Theres plenty out there to help midigate most things. but if your concern of updates (pointless after march 2020) (mostly telemetry and nag patches to upgrade)
I use Windows firewwall control 5 which takes over the default windows firwall as my back end and simplewall as my topical. and between those you can lock out plenty on your system.
Most of the time its programmers and companies who listen to "Microsuck" ("were not supporting windows 7 on March 20th 2020") and everyone jumps on the band wagon and does it too. Its a mental disease really, they really could support all OS'es if they really wanted too. like using the subscriber based windows 7 "security patching" (pay for monthly security patching on windows 7, yet the same patches go into windows 10 automatically via windows update) LAUGHABLE.
Anyway. Learn your system. Download the older Sysinternals form like 2012 and use the hard drive to vhd ap and clone your drive and make a Virtual machine of it.. then play around with settings and firewalls.. (best option).. just keep track of what worked and what broke.. before trying any of this on your system. not alot of options these days, but still plenty to work with. "be your own anti-virus"
only keep things on your computer that you dont mind if someone hacks to get.. always use storage drives as your "secure"
Sorry for the long post. "wifes naggin", for the honey to do list on her vacation and I just ain't that motivated yet this morning.. "stalling for time "Its a important post honey, I got to finish it"" LOL